Process for working a semiconductor wafer

ABSTRACT

A thin adhesive sheet for working semiconductor wafers is described, comprising a light-permeable support and a pressure-sensitive adhesive layer on the support, wherein the adhesive layer is a composition comprising 100 parts by weight of a base polymer, from 1 to 100 parts by weight of a low molecular weight compound containing at least two photopolymerizable carbon-carbon double bonds in the molecule, and from 0.1 to 5 parts by weight of a photopolymerization initiator. This adhesive sheet is firmly bonded to a semiconductor wafer during the wafer cutting process, the adhesive force of the adhesive sheet is markedly reduced. As a result, the pick-up operation can be carried out easily. This adhesive sheet is useful in working semiconductor wafers.

This is a divisional of application No. 07/045,733 filed May 1, 1987,now abandoned, which is a Continuation-in-Part of prior Application No.06/710,828 filed Mar. 12, 1985 (now abandoned).

FIELD OF THE INVENTION

The present invention relates to a thin adhesive sheet for workingsemiconductor wafers. More particularly, it is concerned with a thinadhesive sheet which is used to protect the surface of a semiconductorwafer with IC elements formed thereon during the polishing step, or tofix the wafer when cutting and separating the semiconductor wafer intothe IC element chips.

BACKGROUND OF THE INVENTION

In production of semiconductor wafers, the back of a semiconductor waferwith IC elements formed thereon is generally polished to make the waferas thin and uniform as possible. For example, the thickness of the waferis decreased from about 0.5 mm to from about 0.2 to 0.3 mm by such apolishing.

Various techniques have been employed to prevent the breakage of thesemiconductor wafer or damage of the wafer surface: (a) a methodcomprising coating a paint on the surface of the wafer to form acoating, polishing the back of the wafer and removing the coating with asolvent, (b) a method comprising laminating a thin sheet on the surfaceof the wafer as a spacer, polishing the back of the wafer and removingthe sheet, and (c) a method comprising applying a pressure-sensitivethin adhesive sheet to the surface of the wafer, polishing the back ofthe wafer and peeling off the thin adhesive sheet.

In cutting and separating the semiconductor wafer into IC element chips,a method comprising forming wedge-shaped grooves of low depth on thesurface of the semiconductor wafer in conformity with the shape of thedesired IC chip and dividing the semiconductor wafer into the IC elementchips by applying an external force has been employed. This method,however, has disadvantages in that the separation accuracy is poor, andproductivity is low since the IC element chips after cutting andseparating must be transferred to the subsequent mounting step by thehand.

For this reason, the direct pick-up method has now been employed,comprising fixing the semiconductor wafer by bonding thereto a thinadhesive sheet, cutting the assembly into the IC element chips by meansof a rotary blade and picking up the IC element chips from the thinadhesive sheet while simultaneously mounting the chips.

In the above direct pick-up method, the semi-conductor wafer is washedwith water under a hydraulic pressure of at least 2 kg/cm² to remove afriction heat and scraps during cutting the semiconductor wafer with therotary blade. Thus, it is required for the thin adhesive sheet to have asufficient adhesive force to withstand the hydraulic force. If, however,the adhesive force is too large, it is difficult for the IC elementchips to pick up from the thin adhesive sheet. For this reason, theadhesion force of the thin adhesive sheet is controlled so as to becapable of withstanding the above hydraulic pressure but not to lowerthe efficiency of the pick-up operation.

However, the adhesive force of the thin adhesive sheet can be controlledas described above only when the size of the final IC element chip is upto about 20 mm². In the case of IC element chips having a size of 50 mm²or more as a recent LSI having an increased degree of accumulation, itis difficult for the thin adhesive sheet to control the adhesive forceas described above, and the above-described direct pick-up method cannotbe applied.

SUMMARY OF THE INVENTION

Various investigations have been made to provide a thin adhesive sheetwhich can be used even when the size of the IC element chip is 50 mm² ormore.

Accordingly, an object of the present invention is to provide a thinadhesive sheet for working semi-conductor wafers, comprising alight-permeable support and provided thereon a pressure-sensitiveadhesive layer which can cure by light irradiation, thereby forming athree-dimensional network structure.

DETAILED DESCRIPTION OF THE INVENTION

According to the thin adhesive sheet for working semiconductor wafers ofthe present invention, the adhesive force of the thin adhesive sheet canbe increased to a sufficiently high level without bringing any specificattention to the efficiency of the pick-up operation after the wafercutting process for the reason as described hereinafter. As a result,during the wafer cutting process, IC element chips are firmly bonded tothe thin adhesive sheet and even if pressure of washing water isapplied, the chips never drop from the this adhesive sheet.

After the wafer cutting process, the pressure-sensitive adhesive layeris cured by irradiating with light from the support side of the thinadhesive sheet, thereby forming a three-dimensional network structure.By the formation of the three-dimensional network structure, thecohesive force of the adhesive layer increases and the adhesivenessthereof is substantially lost. As a result, the adhesive force of thethin adhesive sheet to the IC element chips is markedly reduced.Consequently, the pick-up operation can be carried out easilyirrespective of the size of the IC element chip, viz., even if the sizeof the IC element chip is more than 50 mm².

When the thin adhesive sheet of the present invention is used, thedirect pick-up system can be applied even if the size of the IC elementchip exceeds 50 mm² and productivity does never decrease.

Examples of light-permeable support which can be used in preparing thethin adhesive sheet of the present invention include films of syntheticresins such as polyvinyl chloride, polyethylene terephthalate,polyethylene, and polypropylene. These films may be heat-shrinkablefilms. In addition, stretchable films made of polyolefins having rubberelasticity such as polybutene and 1,2-polybutadiene can also be used.The thickness of the film is generally from 10 to 300 μm.

The pressure-sensitive adhesive layer which can cure by lightirradiation to form a three-dimensional network structure is preparedusing a pressure-sensitive adhesive composition comprising a mixture ofa rubber- or acryl-based pressure-sensitive adhesive, a low molecularweight compound having at least two photopolymerizable carbon-carbondouble bonds in the molecule (hereinafter referred to as a"photopolymerizable compound") and a photopolymerization initiator.

The above rubber- or acryl-based pressure-sensitive adhesive is composedof, as a base polymer, rubbers such as natural rubber and varioussynthetic rubbers, or acryl-based polymers such as polyalkyl acrylate ormethacrylate or copolymers comprising from about 50 to 99.5% by weightof alkyl (meth)acrylate and from about 50 to 0.5% by weight of anunsaturated monomer copolymerizable therewith, and if necessary, across-linking agent such as polyisocyanates or alkyl-etherized melaminecompounds, in an amount of from 0.1 to 10 parts by weight per 100 partsby weight of the base polymer. The above base polymers, i.e., rubbers oracryl polymers, may have photopolymerizable carbon-carbon double bondsin the molecule.

The photopolymerizable compound preferably has a number averagemolecular weight of about 10,000 or less. In order that thepressure-sensitive adhesive layer may form a three-dimensional networkstructure with high efficiency by light irradiation, it is morepreferred for the compound to have the molecular weight of about 5,000or less and the number of photopolymerizable carbon-carbon double bondsin the molecule of from 2 to 6, particularly from 3 to 6. Particularlypreferred examples of these photopolymerizable compounds aretrimethylolpropane triacrylate, pentaerythritol triacrylate,pentaerythritol tetraacrylate, dipentaerythritolmonohydroxypentaacrylate, and dipentaerythritol hexaacrylate. Otherphotopolymerizable compounds which can be used include 1,4-butanedioldiacrylate, 1,6-hexanediol diacrylate, polyethylene glycol diacrylate,and commercially available oligoester acrylate.

These photopolymerizable compounds can be used alone or as mixturesthereof. The amount of the photo-polymerizable compound used isgenerally in the range of from 1 to 100 parts by weight per 100 parts byweight of the base polymer. If the amount of the photopolymerizablecompound used is too small, the three-dimensional network structure isformed only insufficiently by the light irradiation of thepressure-sensitive adhesive layer and reduction in the adhesion force ofthe thin adhesive sheet to the IC element chip is too small. On theother hand, if the amount thereof is too large, the plasticity of theresulting pressure-sensitive adhesive layer markedly increases and thenecessary adhesive force cannot be obtained in the wafer cuttingprocess.

Examples of the photopolymerization initiators which can be used includeisopropyl benzoin ether, isobutyl benzoin ether, benzophenone, Michler'sketone, chlorothioxanthone, dodecylthioxanthone, dimethylthioxanthone,diethylthioxanthone, acetophenone diethylketal, benzyl dimethylketal,α-hydroxycyclohexyl phenyl ketone, and 2-hydroxymethylphenylpropane.These compounds can be used alone or as mixtures thereof.

The amount of the photopolymerization initiator used is generally in therange of from 0.1 to 5 parts by weight per 100 parts by weight of thebase polymer. If the amount of the photopolymerization initiator used istoo small, the three-dimensional network structure is formed onlyinsufficiently by the irradiation of the pressure-sensitive adhesivelayer with light and reduction in the adhesive force of the thinadhesive sheet to the IC element chip is too small. On the other hand,even if the amount thereof is increased, any further effect cannot beobtained. Rather, the photopolymerization initiator undesirably remainson the surface of the IC element chip. If necessary, amine compoundssuch as triethylamine, tetraethylpentamine, and dimethylaminoethanol maybe used as photopolymerization accelerators in combination with theabove-described photopolymerization initiators.

The pressure-sensitive adhesive composition composed of theabove-described components is coated on the light-permeable support andthen, if necessary, heated to thereby form the pressure-sensitiveadhesive layer. The thickness of the pressure-sensitive adhesive layeris generally in the range of from about 5 to 70 μm in dry thickness.

The 100% modulus (20° C.) of the pressure-sensitive adhesive layer isgenerally 10 kg/cm² or less and preferably from 0.5 to 8 kg/cm². The gelcontent therein as determined by immersing in toluene (20° C.) for 24hours is usually less than 55% by weight, preferably from 0.5 to lessthan 55% by weight, and more preferably from 35 to less than 55% byweight, and the degree of swelling of gel is generally at least 20 timesand preferably from 25 to 80 times.

In cutting and separating the semiconductor wafer into element chips byusing the thin adhesive sheet of the present invention, comprising theabove-described light-permeable support and pressure-sensitive adhesivelayer, the semiconductor wafer is first adhered and fixed to the thinadhesive sheet and the semiconductor wafer is then subjected to semifullor full cutting using a cutting blade such as a rotary blade. The thinadhesive sheet is irradiated with light having a wave-length range offrom about 180 to 460 nm from the support side thereof for about 2 to180 seconds using a high pressure mercury lamp, a super pressure mercurylamp, or the like. The element chips are then picked up andsimultaneously mounted in the procedure, for example, such that theelement chip is adsorbed with an air pinset simultaneously withapplication of a physical means such as pushing up with a needle.

In the case that the heat-shrinkable synthetic resin film is used as thesupport of the thin adhesive sheet, the sheet can be applied undertension by heating the heat-shrinkable synthetic resin film prior to orafter fixation of the semiconductor wafer, so that the thin adhesivesheet does not deform during the wafer cutting process and thesemiconductor wafer can be cut accurately. In the case that thestretchable synthetic resin or rubber film is used at the support of theadhesive sheet, if the sheet after cutting process is uniformlystretched, fixed, and then irradiated with light under its state, aproper gap is maintained between chips and as a result, the air pinsetis prevented from coming into contact with the neighboring chip, therebyscratching the chip during the pick-up operation.

In these procedures, irradiation with light may be applied on the entiresurface of the thin adhesive sheet, or only on an area where thesemiconductor wafer is bonded to the thin adhesive sheet. In addition, amethod comprising irradiating element chips with light one by onesuccessively and then picking up every chip may be employed.

In polishing the back of the semiconductor wafer using the thin adhesivesheet, the thin adhesive sheet of the present invention is applied tothe front surface of the semiconductor wafer and the back is polished.Thereafter, before peeling the thin adhesive sheet, thepressure-sensitive adhesive layer is cured by irradiating with lightfrom the support side, thereby forming the three-dimensional networkstructure. Due to the formation of the three-dimensional networkstructure, the cohesive force of the adhesive layer increases and theadhesiveness thereof substantially loses, resulting in a markedreduction in adhesive force of the thin adhesive sheet to thesemiconductor wafer. Therefore, regardless of the surface condition ofthe semiconductor wafer, the thin adhesive sheet can be peeled offeasily.

The 180° peeling adhesion force (peeling speed: 300 mm/min) of the thinadhesive sheet to the semiconductor wafer prior to irradiation withlight is generally at least 200 g/20 mm, preferably from 200 to 1,800g/20 mm, and more preferably from 200 to 1,000 g/20 mm. Thus, even undera hydraulic pressure of about 2 kg/cm² which is generally applied duringthe wafer cutting process, the element chips never drop from the thinadhesive sheet.

When the pressure-sensitive adhesive layer is irradiated with light, thephotopolymerizable compound is polymerized and at the same time, freeradicals are generated in the base polymer, and the thus-excited basepolymer reacts with the photopolymerizable compound. As a result, thepressure-sensitive adhesive layer is cured, thereby forming thethree-dimensional network structure.

The term "three-dimensional network structure" as used herein means thatthe gel content as determined by immersing the pressure-sensitiveadhesive layer in toluene (20° C.) for 24 hours is at least about 1.05times, preferably at least about 1.2 times, and more preferably at leastabout 1.4 times the gel content before irradiation with light, and is atleast 55% by weight, preferably at least 60% by weight, and morepreferably at least 70% by weight. The pressure-sensitive adhesive layerirradiated with light has generally the degree of swelling of geldetermined above of 18 times or less and preferably from 5 to 15 times.

Due to the formation of the three-dimensional network structure, thecohesive force of the pressure-sensitive adhesive layer is markedlyincreased as compared with that prior to irradiation with light. The100% modulus (20° C.) is generally increased to at least 20 kg/cm²,preferably from 25 to 150 kg/cm² and more k/cm², and more preferablyfrom 28 to 100 kg/cm². As a result, the adhesiveness of thepressure-sensitive adhesive layer is substantially lost and the adhesiveforce of the thin adhesive sheet to element chips markedly decreases. Atthis time, the 180° peeling adhesion force (peeling speed: 300 mm/min)is generally 150 g/20 mm or less, and preferably from 5 to 125 g/20 mm.Therefore, even if the size of the element chip is more than 50 mm², theelement chips can be easily picked up from the thin adhesive sheet.

The present invention is described in greater detail by reference to thefollowing non-limiting examples. All parts are by weight.

EXAMPLE 1

A composition of 100 parts of butyl acrylate, 5 parts of acrylonitrile,and 5 parts of acrylic acid was copolymerized in toluene to obtain anacrylic copolymer having a number average molecular weight of 300,000.

To 100 parts of the acrylic copolymer were added 5 parts of apolyisocyanate compound (trade name: "Coronate L", produced by NipponPolyurethane Co., Ltd.), 15 parts of dipentaerythritolmonohydroxypentaacrylate, and 1 part of α-hydroxycyclohexyl phenylketone, and they were mixed to prepare a pressure-sensitive adhesivecomposition.

This composition was coated on one surface of a 50 μm thick polyethyleneterephthalate film in a thickness of 10 μm and then dried at 130° C. for3 minutes to obtain a thin adhesive sheet for working semiconductorwafers.

EXAMPLE 2

To 100 parts of the same acrylic copolymer as prepared in Example 1 wereadded 5 parts of the same polyisocyanate compound as used in Example 1,20 parts of pentaerythritol triacrylate, and 0.5 part of isobutylbenzoin ether, and they were mixed to prepare a pressure-sensitiveadhesive composition. This composition was processed in the same manneras in Example 1 to obtain a thin adhesive sheet.

EXAMPLE 3

To 100 parts of the same acrylic copolymer as prepared in Example 1 wereadded 5 parts of the polyisocyanate compound as used in Example 1, 10parts of dipentaerythritol monohydroxypentaacrylate, 1 part ofdimethylthioxanthone, and 1 part of triethylamine, and they were mixedto prepare a pressure-sensitive adhesive composition. This compositionwas processed in the same manner as in Example 1 to obtain a thinadhesive sheet.

EXAMPLE 4

A composition of 100 parts of butyl acrylate and 7.5 parts of acrylicacid was copolymerized in toluene to prepare an acrylic copolymer havinga number average molecular weight of 300,000. Thereafter, the samemanner as in Example 1 was followed except that the above-preparedacrylic copolymer was used as the copolymer, to obtain a thin adhesivesheet.

EXAMPLE 5

A thin adhesive sheet was obtained in the same manner as in Example 1except that 40 parts of 1,6-hexanediol diacrylate was used in place of15 parts of dipentaerythritol monohydroxypentaacrylate.

EXAMPLE 6

A thin adhesive sheet was obtained in the same manner as in Example 1except that 50 parts of a polyfunctional oligoester acrylate (tradename, "Aronix M-8030", produced by Toa Gosei Kagaku Kogyo K.K.) was usedin place of 15 parts of dipentaerythritol monohydroxy pentaacrylate.

COMPARATIVE EXAMPLE

For the sake of comparison, a thin adhesive sheet was obtained in thesame manner as in Example 1 except that 15 parts of dipentaerythritolmonohydroxy pentaacrylate and 1 part of α-hydroxycyclohexyl phenylketone were not used.

TEST EXAMPLE 1

A semiconductor wafer having a diameter of 5 inches was bonded to eachthin adhesive sheet obtained in Examples 1 to 6 and Comparative Example,and the assembly was then cut into element chips having a size of 50 mm²using a rotary blade. This wafer cutting process was carried out whilewashing with water under a hydraulic pressure of 2 kg/cm². In all thethin adhesive sheets, any element chip did not drop.

After the wafer cutting process, the thin adhesive sheet was irradiatedwith light from the support side thereof for 20 seconds using a highpressure mercury lamp (40 W/cm) placed at a distance of 15 cm. Elementchips were picked up by pushing up those using a needle whilesimultaneously adsorbing with an air pinset.

In the thin adhesive sheets obtained in Examples 1 to 6, the elementchips could be easily picked up and furthermore the pressure-sensitiveadhesive layer was not transferred at all to the element chips. On theother hand, in the case of the thin adhesive sheet obtained inComparative Example, the element chips remained bonded firmly to thethin adhesive sheet and could not be picked up.

TEST EXAMPLE 2

180° Peeling Adhesion Force:

The thin adhesive sheets obtained in Examples 1 to 6 and ComparativeExample were measured for the 180° peeling adhesion force (peelingspeed: 300 mm/min) to the semiconductor wafer. Similarly, after the thinadhesive sheet was bonded to the semiconductor wafer and irradiated withlight from the support side thereof under the same conditions as in TestExample 1, the adhesion force was measured.

100% Modulus:

A 50 μm thick polyethylene terephthalate film liner was coated with eachpressure-sensitive adhesive composition in a thickness of 10 μm, heatedat 130° C. for 3 minutes, cut into pieces having a size of 50 mm×50 mm,and collected into a bar-like form to prepare a yarn-like test piecehaving a cross sectional area of 0.5 mm². This test piece was measuredfor 100% modulus at 20° C. Further, another piece cut into a size of 50mm×50 mm was irradiated with light under the same conditions as in TestExample 1. The text piece thus treated was collected into a bar-likeform in the same manner as above and measured for 100% modulus.

Gel Content and Degree of Swelling of Gel:

After coating and heating each pressure-sensitive adhesive compositionin the same manner as in the preparation of the test piece formeasurement of 100% modulus, a test piece having a size of 50 mm×500 mmwas cut out. This test piece was immersed in toluene (20° C.) for 24hours to measure the gel content and the degree of swelling of gel.Further, another piece cut into a size of 50 mm×500 mm was irradiatedwith light the same conditions as in Test Example 1. The test piece thustreated was immersed in toluene for 24 hours to measure the gel contentand the degree of swelling of gel.

The results obtained are shown in the Table below. In the Table, themeasurement values prior to irradiation with light are shown in columnA, and the measurement values after irradiation with light, in Column B.

                  TABLE                                                           ______________________________________                                        180° Peeling                Degree of                                  Adhesion Force                                                                              100% Modulus                                                                             Gel Fraction                                                                            Swelling of                                (g/20 mm)     (kg/cm.sup.2)                                                                            (wt %)    Gel (times)                                Run No.                                                                              A       B      A    B     A    B    A    B                             ______________________________________                                        Example 1                                                                            500     30     2.0  60    50   85   25   10                            Example 2                                                                            530     70     3.0  80    50   85   28   12                            Example 3                                                                            300     25     4.0  85    50   80   26    8                            Example 4                                                                            800     35     0.5  30    40   70   20   10                            Example 5                                                                            340     100    1.5  50    40   65   30   13                            Example 6                                                                            320     120    2.0  55    40   65   35   15                            Compara-                                                                             350     800    2.0  3.0   52   55   23   20                            tive Ex-                                                                      ample                                                                         ______________________________________                                    

EXAMPLE 7

The thin adhesive sheet obtained in Example 1 except that the thicknessof the adhesive-lay was 30 μm was bonded to the surface of asemiconductor wafer having a diameter of 5 inches and a thickness of 0.5mm with IC elements formed on the surface thereof. The of the wafer waspolished using surface grinder. In this polishing, the semiconductorwafer was neither broken nor cracked, and the surface of thesemi-conductor wafer was not scratched. Moreover, the permeation ofwater between the semiconductor wafer and the adhesive sheet did notoccur.

After the polishing process, the adhesive sheet was irradiated withlight for 20 seconds from the polyethylene terephthalate film side usinga high pressure mercury lamp (40 W/cm) placed at a distance of 15 cm andthen peeled off from the semiconductor wafer. This peeling could beperformed very easily, and the transfer of the pressure-sensitiveadhesive layer to the surface of the semiconductor wafer was notobserved at all. The thus-polished semiconductor wafer had a thicknessof 0.25 cm and an entire uniform thickness. The 180° peeling adhesiveforce of the adhesive sheet was 350 g/20 mm prior to irradiation withlight and 35 g/20 mm after irradiation with light.

It can be seen from the above results that when the thin adhesive sheetof the present invention is used in working the semiconductor wafer,element chips obtained by cutting the semiconductor wafer are firmlybonded to the thin adhesive sheet and never drop from the thin adhesivesheet during the wafer cutting process, and after the wafer cuttingprocess, the element chips can be easily picked up by irradiation of thethin adhesive sheet with light from the support side even though thesize of the element chip is more than 50 mm². Furthermore, in thepolishing step, the semiconductor wafer can be polished efficiently andthe thin adhesive sheet can be easily peeled off.

The reason why the element chips can be easily picked up and afterpolishing, and the thin adhesive sheet can be easily peeled off can beunderstood from the fact that the cohesive force of thepressure-sensitive adhesive layer markedly increases due to theformation of the three-dimensional network structure by irradiation ofthe thin adhesive sheet with light and as a result, the adhesive forceof the thin adhesive sheet to the element chips markedly decreases.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A process for working a semiconductor wafer,which comprises the steps ofadhering and fixing to the surfacedesignated the front surface of a semiconductor wafer a thin adhesivesheet for working the semiconductor wafer comprising a light-permeablesupport and a pressure-sensitive adhesive layer provided thereon whichcures by irradiation with light to form a three-dimensional networkstructure, has a 180° peeling adhesive force to a semiconductor wafer of200 g/20 mm or more at a peeling speed of 300 mm/min, and is comprisedof 100 parts by weight of a base polymer, from 1 to 100 parts by weightof a low molecular weight compound containing at least twophotopolymerizable carbon-carbon double bonds in the molecule, and from0.1 to 5 parts by weight of a radical photopolymerization initiator;polishing the surface of said wafer opposite to and parallel with saidfront surface designated the back surface, to which the adhesive thinlayer is not adhered; and irradiating said front surface of said waferwith light to cure the pressure-sensitive adhesive layer and form athree-dimensional network structure, thereby decreasing the peelingadhesive force to 150 g/20 mm or less.
 2. The process of claim 1 whereinsaid low molecular weight compound has a number average molecular weightof about 10,000 or less and the number of photopolymerizablecarbon-carbon double bonds is 2-6.
 3. The process of claim 1 whereinsaid low molecular weight compound has a number average molecular weightof about 5,000 or less and the number of photopolymerizablecarbon-carbon double bonds is 3-6.
 4. A process for working asemiconductor wafer, which comprises the steps ofadhering and fixing tothe surface designated the back surface of a semiconductor wafer a thinadhesive sheet for working the semiconductor wafer comprising alight-permeable support and a pressure-sensitive adhesive layer providedthereon which cures by irradiation with light to form athree-dimensional network structure, has a 180° peeling adhesive forceto a semiconductor wafer of 200 g/20 mm or more at a peeling speed of300 mm/min, and is comprised of 100 parts by weight of a base polymer,from 1 to 100 parts by weight of a low molecular weight compoundcontaining at least two photopolymerizable carbon-carbon double bonds inthe molecule, and from 0.1 to 5 parts by weight of a radicalphotopolymerization initiator; cutting the semiconductor wafer parallelwith said back surface to form a surface designated the front surface;and irradiating said back surface with light to cure thepressure-sensitive adhesive layer and form a three-dimensional networkstructure, thereby decreasing the peeling adhesive force to 150 g/20 mmor less.
 5. The process of claim 4 wherein said low molecular weightcompound has a number average molecular weight of about 10,000 or lessand the number of photopolymerizable carbon-carbon double bonds is 2-6.6. The process of claim 4 wherein said low molecular weight compound hasa number average molecular weight of about 5,000 or less and the numberof photopolymerizable carbon-carbon double bonds is 3-6.